No RPM signal with MS3Pro and HEI 8

[McMatt]

I'm installing an MS3Pro on a SBC with an HEI 8 ignition. I am going through the pre-start checks in the manual and when I got to the point of verifying RPM I got no reading for RPM in TunerStudio when cranking the engine. I ran the tooth logger and got nothing there either. I hooked up an oscilloscope to CKP+ and Sensor return right at the 35 pin connector. When I cranked the engine I was able to see a clean square wave coming off of the HEI module on the scope. I'm stumped as to why the MS3Pro isn't seeing any RPM.

Here is how I have it wired up

MS3Pro HEI8 Wiring diagram.png

Here is the square wave I'm seeing on the oscilloscope

IMG_4484 copy.jpg

[Matt Cramer]

Can you check the black CKP- wire? Make sure it's not grounded; it needs to be at 2.5 volts.

[McMatt]

I have the black wire folded back and covered in heat shrink so it shouldn't be grounded but I'll open up the heat shrink and make sure it isn't touching the shielding wire. I'll check it out tonight.

[fuelie74]

Sounds like we are both having the same problem. Mine will read the rpm, start and run if I disconnect pin b from the distributor.

http://www.msextra.com/forums/viewtopic ... 77#p406877

[McMatt]

Interesting. I am going to go try disconnecting pin B right now and see if I get RPM signal. brb

[McMatt]

Sure enough. I disconnected pin B and I started seeing RPM in TunerStudio.

[fuelie74]

Could it be something simple like having to go into the the programmable on/off outputs and set up ign b?

[fuelie74]

Not sure what the fix is but I think I know what the problem is.

Hooked up my DVOM to the Pin B wire with it disconnected from the distributor. While cranking the engine and for a few seconds after it started I had 5V coming from the ECM then in went to zero. If I am reading this correctly it should be the opposite.

In a GM vehicle, the override signal is zero volts during cranking (less than 400 rpm or 5 to 15 seconds), then there is 5 volts on this wire after the engine starts to signal the 7-pin module that it should use the signal on the white wire to control timing. You need to apply a 5 volt signal here in order to control the timing while running. A wire can be run from the 5 Vref for the TPS (Pin 26 on the DB37) to the HEI pin B through a relay that is switch from a source that is hot in RUN but not CRANK (see the diagram below). Most cars have a source like this, check your service manual.

http://www.megamanual.com/ms2/GM_7pinHEI.htm

[jsmcortina]

I'll double check the bypass operation next week. MS3-Pro flips the polarity in hardware, so it _might_ be backwards compared to MS3 in error.

James

[McMatt]

Matt ,

I verified that CKP- is not grounded and I saw 2.4v on that wire when I hooked it up to a voltmeter. Still not seeing RPM when the B wire is connected.

[fuelie74]

Any chance you have looked at the bypass operation yet?

[jsmcortina]

I sent some code to Matt for him to try out.

James

[Matt Cramer]

I have just confirmed James's new code fixes the issue.

[fuelie74]

That is awesome. What do we have to do to get it?

[jsmcortina]

That is awesome. What do we have to do to get it?

PM me your email address.

James

[fuelie74]

Address sent

[McMatt]

Sent my e-mail address as well.

[McMatt]

I just loaded up the 1.3.2beta2 firmware and the fix provided by James on my MS3-Pro. Looks like the flipped bypass output was causing the problem with the RPM signal. With this fix I was able to get an RPM signal with the bypass connected and disconnected. Thanks for looking at this so quickly.

Are these changes going to be pushed into a point release of the 1.3.x branch?

[jsmcortina]

I just loaded up the 1.3.2beta2 firmware and the fix provided by James on my MS3-Pro. Looks the flipped bypass output was causing the problem with the RPM signal. With this fix I was able to get an RPM signal with the bypass connected and disconnected. Thanks for looking at this so quickly.

Are these changes going to be pushed into a point release of the 1.3.x branch?

Yes. That's why the code is called "pre-1.3.2 ...." It is a test code leading up to 1.3.2. (Due tomorrow.)

James

[McMatt]

Awesome. Thanks James. I'm new to the Megasquirt world so I wasn't sure how long it took to get changes pushed out to a release.